The present invention relates to a light-sensitive printing plate that comprises a layer carrier, a light-sensitive layer provided on the carrier, and an ink-repellent layer, comprised of a silicone elastomer, which is superimposed on the light-sensitive layer.
Printing plates that include a silicone elastomer, ink-repellent layer are known, for example, from German Offenlegungsschrift No. 1,671,637. The light-sensitive layers used in such plates are preferably those based on polycondensation products of aromatic diazonium salts and formaldehyde. Upon exposure, these layers become insoluble in the developer and can be developed with aqueous solutions. During development, those areas of the silicone elastomer layer which are located above the soluble, unhardened areas of the light-sensitive layer are removed together with these soluble areas, even though the elastomer-layer areas are insoluble in the developer. It is necessary, however, to add sufficient quantities of organic solvents to the developer, so that the silicone elastomer exposed to the developer becomes swollen, and thus softened. For example, mixtures of isopropanol and water are used for development.
The silicone elastomers used for this purpose are single-component elastomers which, for hardening after application of the layer, require moisture and take a considerable length of time. Such single-component silicone elastomers are usually polysiloxanes; for example, dimethylpolysiloxanes, which contain terminal acetyl, oxime, hydroxyl or amino groups.
German Offenlegungsschrift No. 2,323,972 discloses a similar plate with a single-component silicone elastomer, wherein the light-sensitive layer contains (1) a reaction product obtained from a diphenylamine-4-diazonium salt/formaldehyde condensation product and 2-hydroxy-4-methoxy-benzophenone-5-sulfonic acid; and (2) a binder, if appropriate. Since the solubility of the diazonium salt precipitation product is very low, both the preparation and the development of the plate are made considerably more difficult.
In German Offenlegungsschrift No. 2,357,871, presensitized printing plates for waterless offset printing are also disclosed which contain a multicomponent silicone elastomer layer. In addition to organopolysiloxanes, these layers contain, for example, low-molecular silanes which are capable of reacting with functional end groups of the polysiloxanes to effect crosslinking in the absence of moisture. These multicomponent systems can crosslink by means of addition reactions or condensation reactions. In the addition type, for example, terminal alkenyl groups react with Si-H groups in the presence of catalysts at an elevated temperature. In the condensation-crosslinking systems, terminal functional groups of polydiorganosiloxanes react with functional groups of silanes or oligosiloxanes which act as crosslinking agents, in most cases in the presence of catalysts.
The printing plates obtained with these silicone elastomer layers show good ink repellency in the non-image areas and good resistance of the ink-repellent silicone layer. The best image resolutions can be obtained with the diazonium salt layers that are preferred as the light-sensitive layers. But these layers, which are in most cases highly hydrophilic, are difficult to combine precisely with the particularly ink-repellent and resistant silicone layers that are preferred. As a consequence, it is frequently impossible to find a solvent combination that resolves the non-image areas of the diazo layer without attacking the image areas and, simultaneously, that fully and cleanly removes the silicone elastomer layer from the non-image areas.